1. Field of the Invention
The present invention generally relates to the investigation and characterization of geologic formations, and more particularly to a method and system for modeling geological objects, or geological bodies, in regions which have a deformation such as folding, faulting, fracturing, shearing, compression, or extension.
2. Description of the Related Art
Geologic data are used for land-management decision-making, engineering design, in the hunt for mineral resources, and for scientific research. Geologists have devised a wide variety of techniques to collect and analyze data relating to the structure and content of earth formations in the continuing search for underground assets, particularly hydrocarbons such as oil and gas. These techniques include, for example, seismic sensing, and downhole logging. In seismic sensing, a sound source is placed at the surface, or at an underground location, and an array of seismic sensors collect information on the resulting sonic waves. In downhole logging, instruments (e.g., magnetic induction sensors or gamma-ray sensors) are attached to a wellbore tool that transmits sensed data up the wireline or via another communication channel to a data processing system. Analysis of the information found using these different techniques reveals the structures of subsurface formations, and the nature of the formations, i.e., porosity, density, etc., all of which is useful in determining the rock constituents and whether hydrocarbons are present.
Analysis of geologic data often exposes underground structures such as fluvial channels and levees, windblown dune sand bodies, or reef structures. These various sedimentary features are commonly referred to as geological bodies, also known as geological objects. More generally, geological bodies are three-dimensional depositional structures in subsurface geology, which are more localized than the remainder of the depositional formations. It is known to model geological bodies mathematically (particularly using computer programs) in a three-dimensional structural model by a closed three-dimensional boundary surface. Modeling of subsurface structures can assist in the search for and extraction of underground assets. For example, flow behavior, connected volume and overall performance of hydrocarbon reservoirs are all highly dependent on the petrophysical properties of geological bodies.
An important concept in analyzing the information contained in geologic models is the distinction between a description of a rock volume, and a description of a surface. Rock units describe the characteristics of a volume of rock. Surficial geologic units describe the characteristics of the boundary layer between rock volumes with different properties or between solid earth and the atmosphere or the hydrosphere. Surficial units may describe the lithology of deposits to a depth that is small relative to the horizontal extent of the model, or may relate to surface morphology, age (as opposed to deposit age), or depositional environment. To a geologist interested in the processes and characteristics of the earth subsurface, the surfaces in the model represent boundaries of volumes in the model. A geologist interested in the rock bodies that compose the earth uses the three dimensional geometry of the boundary surfaces, to understand the formation.
Geological bodies may be found in a region having some deformation, such as that caused by faulting. In such a case, a structural model might consist of several three-dimensional fault blocks delimited by fault surfaces and, within the fault blocks, block units further delimited by depositional horizons and unconformities. As used herein, a depositional horizon, or horizon, is a surface delimiting depositional rock volumes; and an unconformity refers to an erosional surface.
A geologist requires an understanding of relevant deformation processes that have affected a region. Deformation processes include the growth of folds or faults in three dimensions, as well as developed spatial relationships between the deformation and sedimentation.
Accurate characterization and modeling of geological objects requires an understanding of the shape and location of the objects at the time of their deposition prior to folding and faulting. A deposition-time model, or a model of a geological object at the time of deposition, is called a paleo-space model. Once a geological object has been modeled in paleo space, it is necessary to transform the geological object from paleo space to the contemporary space and morphology; in particular, the deformation (e.g., folding and faulting) known to have affected the contemporary setting must be applied to the objects modeled in paleo space. The transformation between paleo space and contemporary space is necessary to determine an estimate of surface strains, displacements and faults so that the contemporary shape of the geological bodies can be extrapolated from the spatially limited samples represented by well log data. Present techniques for manual and statistical generation of geological bodies support their construction only in unfaulted settings (e.g., layer-cake models). In faulted settings, a geological body may have to be modeled multiple times, once in each containing fault block, with a different deformed shape in each block. Currently, no tool available permits modeling of folding after the deposition of the geological body, except for the FluvSim™, a fluvial simulation geostatistical package available in Modeling Office, GeoFrame 4.0™.
One tool that is used for modeling geologic formations is the Geoframe™ Modeling Office marketed by Schlumberger. As implemented in the Geoframe™ GF4 Modeling Office, geological bodies are first constructed in the contemporary setting, thereafter deformed to conform to a datum horizon, and then trimmed using a non-destructive focused classify operation to fit within a specified block unit. Defining a geological body that spans multiple block units remains tedious because the geological body must be remodeled for each block unit. This is problematic when a three dimensional geological body has a lateral extent spanning multiple fault blocks in a three dimensional structural model. In cases spanning multiple fault blocks, prior art methods require that the feature must be modeled separately in each fault block. In prior art methods, each fault is extrapolated past the boundaries of the fault block and a classify technique is applied. The classify technique compares two sets of geometries to classify the points of the one set with respect to the points of the other according to whether the points of one geometry are inside, on, or outside the other geometry. According to the focused classify technique, a particular sub-volume of a model, for example a particular block unit or fault block is focused upon, as opposed to a classify of a surface or geological object against all of the volumes in a model. This “focused classify” of the extrapolated fault surface is performed upon a fault block that is a target of investigation, referred to as a “target fault block.” As a result of a focused classify one or more split fault blocks can be further subdivided by additional faults. Limited post-depositional deformation of the geological body can be captured by making the geological body shape conform to one or two controlling surfaces, but these must be single-valued height fields so general deformations are not supported in the current art.
There are many approaches to restoring geological horizons in two-dimensional section or map views, or in three dimensions. The three-dimensional techniques, in particular, allow the user to derive a paleo-space model from a three dimensional structural model. Most approaches to paleo-space modeling (also referred to as “palinspastic reconstruction”) are focused on building balanced section views of the paleo-space model by transforming corresponding two-dimensional sections of the contemporary model. These section views can be interpolated to provide a corresponding three dimensional paleo-space model, but such interpolation is often inaccurate, particularly with regard to strike-slip movements perpendicular to the plane of the section. Another tool, the GeoQuest™ GeoViz™ system (also marketed by Schlumberger, Inc.), supports the flattening of three-dimensional seismic data on a given horizon for visualizing and interpreting seismic data in a three-dimensional setting. However, the transformation is not applied to faulted structural models. GeoViZ™ advantageously combines geophysical, geological, petrophysical and reservoir data, allowing the viewing of a true perspective of geospatial relationships.
One recent publication which addresses the restoration of folded and faulted three-dimensional models is “3-D Restoration of Complexly Folded and Faulted Surfaces Using Multiple Unfolding Mechanisms,” Rouby et al., Amer. Assoc. Petroleum Geologists Bulletin v. 84, no. 6, pp. 805-829 (June 2000). The method therein described performs restorations on sets of stratigraphic horizons defined in three dimensions as irregular triangular networks (triangulated surfaces), with the unfaulting and unfolding as separate steps. Starting at the deformed state, the method first unfolds the horizon by choosing among three deformation mechanisms. After unfolding, unfaulting is performed in a map view. Before unfaulting, normal faults appear as gaps separating fault compartments. To invert the displacement on the fault, the gaps are closed by rigid-body motion of the fault compartments. The difference between the deformed and the restored state gives the three-dimensional finite displacement field and the directions of slip on the faults. Another approach to restoration is disclosed in “Unfolding a Horizon: New Capabilities and Applications,” Levy et al., GOCAD Consortium Annual Meeting (June 2000). According to that technique, a surface is unfolded based on specific surface parameterization. The parameterization of a surface is a one-to-one transform function putting a surface in a three dimensional domain in correspondence with a surface in a two dimensional domain. The Move3D (Midland Valley Consultants) system provides restoration techniques for folded and faulted models and supports paleo-transformation of data from geological measurements. The system also provides inverse paleo-transformation from the paleo-space model to the contemporary structural model. Although any of these approaches is suitable for the construction of a paleo-model, none provides a mechanism for modeling of deformation after the deposition of a geological body and focused classify of the deformed body.
In light of the foregoing, it would be desirable to devise an improved method of modeling a geological object in a formation that has been deformed by, e.g., faulting and folding. It would be further advantageous if the method could preserve the topology of the volume entities, and consistently transform all data positioned on or in the volume entities when transforming from contemporary to paleo space.